[Engine-XMPP] prevent Threaded parts from accessing XmppProtocolManager without lock or after Disposed has been called
[Engine-XMPP] not yet synced chats do not need to be disabled
[Engine-XMPP] removed unreachable code

ContactChat is already disabled with all the other chats, therefore IsEnabled is false.
Merge pull request #113 from oli-obk/fix/xmpp_threads